In a semiconductor device production process, as the performance of a semiconductor device is improved, techniques for producing the wiring with higher density and higher integration are required. In a production process of such a semiconductor device, chemical mechanical polishing (CMP) is an essential process. As the miniaturization of the semiconductor circuit progresses, it is demanded to realize the high flatness required for the unevenness of a pattern wafer and also to realize the high smoothness of nano order by CMP. In order to realize the high smoothness by CMP, it is preferred that the convex portion of the pattern wafer is polished at a high polishing rate but the concave portion is not polished so much.
Herein, in a case of using a pattern wafer made of a silicon nitride film (SiN film), since the silicon nitride film usually has unevenness, when polishing such a material, not only the convex portions but also the concave portions are scraped together, and the unevenness are hardly sufficiently eliminated.
In addition, the semiconductor wafer is constituted of dissimilar materials including polycrystalline silicon forming a circuit, silicon oxide being an insulating material, and silicon nitride for protecting a silicon dioxide surface that is not part of the trench or the via from the damages during etching. Therefore, a phenomenon such as dishing, in which a material that is relatively soft and easily reacts with a polishing agent, such as polycrystalline silicon, and silicon oxide is scraped excessively as compared with the silicon nitride or the like surrounding the material, is generated, and unevenness is left.
From these things, in a polishing process of a pattern wafer made of a hard and chemically stable material such as silicon nitride, it is required to sufficiently eliminate the unevenness.
As a technique to respond to the requirement, for example, in JP 2012-040671 A, for the purpose of providing a polishing composition capable of polishing at a high speed an object to be polished that is poor in chemical reactivity, such as silicon nitride, a technique in which colloidal silica which is immobilized with an organic acid (sulfonic acid-modified aqueous anionic silica sol) is allowed to be contained in the composition as abrasive grains, and the pH is adjusted to 6 or less has been disclosed.
Herein, in general, there is a problem that silica sol such as colloidal silica is unstable because silica particles aggregate with each other under an acidic condition. As a technique to solve such a stability problem, in JP 2010-269985 A, sulfonic acid-modified aqueous anionic sol having a zeta potential of −15 mV or less at an acidic of pH 2 or more has been disclosed. In addition, in JP 2010-269985 A, as a method for producing the anionic sol described above, a technique in which a silane coupling agent having a functional group chemically convertible to a sulfonic acid group (for example, a mercapto group) is added to colloidal silica, and then the functional group is converted to a sulfonic acid group has been disclosed. Herein, in Examples of JP 2010-269985 A, silica sol containing water and methanol as a dispersing medium is heated and concentrated under alkaline and normal pressure conditions, and then into the silica sol, a mercapto group-containing silane coupling agent (3-mercaptopropyl trimethoxysilane) is added, and the resultant mixture is refluxed at a boiling point and heat aged. Next, methanol and ammonia are replaced with water, and the resultant mixture is cooled down to room temperature at the time point when the pH becomes 8 or less, and into the cooled mixture, hydrogen peroxide water is added, the mercapto group is converted to a sulfonic acid group by heating the mixture, as a result, anionic silica sol of which the surface has been modified with a sulfonic acid group is obtained.
In addition, in JP 2013-41992 A, there is a disclosure about the production of similar sulfonic acid-modified aqueous anionic silica sol, referring to the above-described JP 2010-269985A, and J. Ind. Eng. Chem., Vol. 12, No. 6 (2006) 911-917. Herein, in Examples of JP 2013-41992 A, into an aqueous solution of the mercapto group-containing silane coupling agent similar to that as described above (under an acidic condition with acetic acid), silica sol containing water as a dispersing medium is added, the resultant mixture is stirred at room temperature for one hour, and then into the mixture hydrogen peroxide water is added, and the resultant mixture is left to stand at room temperature for 48 hours, as a result, sulfonic acid-modified aqueous anionic silica sol is obtained.